1. Field of the Invention
The present invention relates to a method of producing a thin semiconductor film for use in the field of electronics, and a plasma CVD (Chemical Vapor Deposition) apparatus to be used for producing the same. In particular, the present invention relates to a method of producing a thin semiconductor film suitably used for optical semiconductor devices such as solar batteries, and a plasma CVD apparatus to be used for producing the same.
2. Description of the Related Art
Today, methods for producing thin films from a vapor by utilizing plasma are applied to various fields, e.g., metal films, semiconductor films, and dielectric films. In particular, plasma CVD devices are employed to place a group-IV hydrogen compound into a plasma state, thereby decomposing the group-IV compound into active species, which are deposited on a substrate. Vigorous research activity has been conducted on hydrogenated amorphous silicon type thin films formed by this method, such as hydrogenated amorphous silicon (a-Si: H), hydrogenated amorphous silicon carbon (a-SiC: H), and hydrogenated amorphous silicon germanium (a-SiGe: H)), since its PN controlling properties (which is one of the most important properties of semiconductor materials) were reported. Such thin semiconductor films have been employed in practical applications such as solar batteries for consumer use (e.g., those for calculators), the core sections of liquid crystal display devices, photosensitive drums, and various sensors, and the like.
In order to further broaden the application range of devices incorporating such thin semiconductor films and to expand the market thereof, it is desired to find a method for rapidly forming a high-quality film with a high yield. In the case of amorphous solar batteries incorporating hydrogenated amorphous silicon type thin films, for example, they are desired to be employed in solar-battery power supply applications, as well as the above-mentioned consumer use, e.g., calculators. In order for such applications to be practical, however, the production cost of the thin semiconductor films must be minimized for possible competition with existing commercial power supplies.
According to a cost analysis by the present inventors, in order to sufficiently reduce the production cost of hydrogenated amorphous silicon type thin films for competition with existing commercial power supplies, the film formation rate of hydrogenated amorphous silicon type thin films must be enhanced by one digit (e.g., 600 angstroms/min) from the currently available rate. Furthermore, capability of depositing high-quality films with a high yield is required.
However, although the plasma CVD method can achieve a higher-rate film formation by supplying a larger high-frequency power and/or supplying a larger amount of material gas, if semiconductor films were formed at a high rate, e.g., 300 angstroms/min, by such a method, a large amount of polysilane powder would be generated, thereby resulting in a low yield. (Hereinafter, this method will be referred to as "conventional plasma CVD method 1").
On the other hand, there has been proposed a method for rapidly forming a-Si: H films utilizing a plasma CVD method which can solve the above-mentioned problem of powder generation (Apply. Phys. Lett. 53(1988)1263, Appl. Phys. Lett. 57(1990)1616, and Applied Physics, vol. 52, No. 7(1993)699). This plasma CVD method employs a modulated high frequency power supply as an excitation power supply which repeats a periodical switching on and off, as shown in FIG. 1A, in which a high frequency generation circuit and an oscillation circuit which repeats a periodical switching on and off are combined. (Hereinafter, this method will be referred to as "conventional plasma CVD method 2").
Conventional plasma CVD method 2 is known to remarkably reduce polysilane powder generation within the reaction apparatus (as compared with conventional plasma CVD method 1) when depositing an a-Si: H film under the following conditions: a periodic on-off frequency of approximately 1 kHz; a duty ratio of approximately 50% (which provides an ON period of 500 .mu.sec); a film formation rate of approximately 300 angstroms/min to 600 angstroms/min. If any undesirable powder deposited in the reaction apparatus is attached to a substrate, more frequent maintenance activities for the apparatus will be required, and the production yield will decrease. In this regard, conventional plasma CVD method 2 is recognized to increase the throughput of the production line and the production yield by reducing the amount of powder generated in high-rate film formation.
However, it had not been specifically examined whether or not a thin film formed by conventional plasma CVD method 2 (described above) is suitable as a thin semiconductor film for photosemiconductor devices such as solar batteries.